The present invention relates to a metal interconnect structure including a test pad structure that enables access to test structures at various levels by repeated use of a same mask set, methods of forming the same, and a mask set for effecting the manufacture of the same.
Test sites refer to a mask set including patterns for manufacturing a semiconductor chip including testable devices. Test sites are typically designed to include a predetermined number of metal levels. To qualify a new generation of technology, however, multiple types of builds having different numbers of metal levels are sometimes necessary.
Masks are expensive and account for a significant fraction of the total development cost in qualifying new technology. If mask sets for all optional build levels are fabricated, the cost of the additional mask sets are added to the cost of the basic mask set as development cost. Nonetheless, qualification of the new technology requires building hardware including the optional metal levels and testing the functionality of test structures in such optional levels.
If the same masks are employed more than once, this configuration results in parallel structures within which a lower level structure is electrically shorted to an upper level structure having the same pattern. For example, masks for the basic levels can be employed more than once to build hardware including optional levels by using the masks for the basic levels to pattern structures in the optional levels. However, any test structure within one of the optional levels (which is typically the upper levels) is electrically shorted to a congruent test structure within one of the basic levels because the patterns are the same if conventional test pad structures are employed. Consequently, any attempted testing of a test structure in one of the optional levels necessary results in testing of two test structures in parallel, i.e., testing of two test structures that are electrically shorted to each other trough overlying test pads. Thus, reuse of conventional mask sets is an inadequate method for testing structures in optional metal levels.